Frictional coupling in a vehicle between a tire and a road surface is important for safe travelling of the vehicle. This is because for any dynamic process (accelerating the vehicle by operating an accelerating pedal and decelerating the vehicle by operating a brake pedal, etc.) it is necessary to transmit between the tire and the road surface a force corresponding to the dynamic process. Frictional coupling between the tire and the road surface depends on specific slipping of a wheel, and when the specific slipping exceeds a specified range, frictional coupling is weakened and the vehicle becomes unstable.
Controllers for preventing a specific slipping rate of a vehicle from exceeding a specified range at times of decelerating or accelerating a vehicle are known. In these controllers, rotational speed and acceleration of a wheel are detected by means of a wheel sensor, and based on values thus obtained and other measured values, a wheel specific slipping rate etc., are calculated in an electronic evaluation controller. Antiskid controllers have been devised wherein braking pressure is reduced by a suitable controller when wheel slipping exceeds a specified range and braking pressure is increased when wheel speed is restored and the amount of wheel slipping becomes acceptable, so that braking distance is shortened by obtaining a vehicle braking force that is as strong as possible.
In the above-described antiskid controller, if the level of frictional coupling between the wheel tire and the road surface is known prior to starting of control, a control suited to the level of coupling between the tire and the road surface can be carried out from the start of the control. That is, where the level of coupling between the tire and the road surface differs from known values, in other words road surface friction coefficient becomes different, control of brake liquid pressure accompanying movements of wheels may be different based on wheel speed reduction timing differences between front and rear wheels.
Thus, for instance Japanese Unexamined Patent Publication No. 4-345562 describes a method for executing antiskid controls by obtaining a wheel speed difference between the front and rear wheels of the vehicle, estimating whether a level of friction on the road surface is low (hereinafter called low .mu. road) or high (high .mu. road) based on a relationship between the wheel speed difference and the duration of a state in which the difference exists and using a result of the estimation for antiskid control.
Such an antiskid control method, however, only determines whether the road surface on which the vehicle travels is a high .mu. road or a low .mu. road based on the relationship between the speed difference between front and rear wheels and the duration of the speed difference, and not on specific road surface friction coefficients corresponding to continuously changing road surface situations. Therefore, for example, only determination of low .mu. roads and high .mu. roads can be realized, and a medium .mu. road surface including a wet road, a compressed snow road or the like must simply be determined as either the high .mu. road surface or as the low .mu. road surface. If the vehicle is controlled based on this kind of determination, vehicle control may become unstable. A vehicle body speed is used as a reference for determining the above-described wheel slipping ratio. If the vehicle body speed is not correctly calculated, the starting time of the antiskid control may not be accurately determined or control of brake fluid pressure may not be accurately executed. Thus, an optimum braking force cannot be obtained. In this kind of antiskid controller, usually the vehicle body speed of a two-wheel-drive vehicle is estimated from the wheel speeds of four wheels, and if braking hydraulic pressure is reduced by means of ABS control after starting braking of the vehicle, the reduced speed of a rolling wheel is restored to the vicinity of the actual vehicle body speed. Therefore, an estimated vehicle body speed calculated from the wheel speeds of four wheels is almost equal to the actual vehicle body speed, and thus, nearly accurate ABS control performance can be obtained. In a case where more accurate control is desired, even in ABS for the two-wheel-drive vehicle, an attempt has been made to estimate an accurate vehicle body speed by adding an acceleration sensor for detecting acceleration and deceleration of a vehicle body or by dividing road surface friction coefficients into categories by using the method described in the above-described publication.
On the other hand, in the case of a four-wheel-drive vehicle, four wheels are simultaneously driven by means of mechanical coupling of a differential gear and are greatly affected by engine torque and transmission torque. Thus, even though braking hydraulic pressure is reduced by means of ABS control after a start of braking, a wheel speed may not be restored to the vicinity of the real vehicle body speed as in the case of the two-wheel-drive vehicle. Rather, estimated vehicle body speed deviates from the real vehicle body speed and, particularly on a road surface where a surface friction coefficient (road surface .mu.) is extremely low (extremely low .mu. road) such as on an icy road and the like, the difference between estimated and actual speeds becomes greater. In this case, a reference speed and the like for reducing pressure are set so low that slipping of the wheels increases.
Given this situation, for a four-wheel-drive vehicle, the road surface .mu. is estimated by means of an additional sensor, such as a G sensor, for detecting deceleration of the vehicle body. Then, based on this road surface .mu., optimizing ABS control is carried out by switching the estimated vehicle body speed and various control references to values suited to the estimated road surface .mu..
Therefore, it is apparent that the additional sensor such as the G sensor for detecting deceleration of the vehicle speed is essential to ABS for four-wheel-drive vehicles compared with other systems for two-wheel-drive vehicles. The system is specialized for the four-wheel-drive vehicle, not only causing an increase in cost but also necessitating a different control from that for the two-wheel-drive vehicle, and a common system for all kinds of vehicles is not possible.
The present invention is made in view of these problems and it is an object of the invention to provide an antiskid controller wherein, without using an additional sensor such as an acceleration sensor (or a G sensor), a road surface .mu. can be correctly estimated in accordance with changing conditions of road surfaces and antiskid control can be accurately executed by using the estimated value.